Molecular Dynamics Simulation of Crystallization Cyclic Polymer Melts As Compared to Their Linear Counterparts
HY Xiao and CF Luo and DD Yan and JU Sommer, MACROMOLECULES, 50, 9796-9806 (2017).
Large scale molecular dynamics simulations have been performed in the framework of a coarse-grained poly(vinyl alcohol) model to study the crystallization behavior of long unknotted and nonconcatenated cyclic polymer chains from the melt state. The results are compared with those for chemically identical linear chains. The crystallization and melting points, stem length, crystallinity, and latent heat of melting/crystallization of cyclic polymers are found to be substantially higher than their linear counterpart subjected to the same thermal history. The reduced amount of entanglements of cyclic polymers is suggested to explain the difference between cyclic polymers and their linear analogue. We applied primitive path analysis to quantify the entanglement state for all systems during crystallization and heating. While for linear chains the entanglement length is increasing during crystallization and annealing indicating a partial disentanglement process, the entanglement length is monotonously decreasing after the onset crystallization for cyclic polymers. We suggest that segments which are trapped by the formation of lamellar crystals essentially contribute to the entanglement density in crystallizing cyclic polymers. The increase of stem length (reorganization) during heating, as found in linear system, is not shown in cyclic polymers, in agreement with recent experimental observations Zardalidis et al. Soft Matter 2016, 12, 8124. This correlates with the observation of creation of entanglement constraints during crystallization in our simulations.
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